Re: [Qemu-devel] [PATCH 0/9] introduce virtio net dataplane

[Anthony Liguori]

"Michael S. Tsirkin" <address@hidden> writes:

> On Thu, Feb 21, 2013 at 10:15:58AM -0600, Anthony Liguori wrote:
>> "Michael S. Tsirkin" <address@hidden> writes:
>> 
>> > On Thu, Feb 21, 2013 at 08:54:44PM +0800, Liu Ping Fan wrote:
>> >> This is a emulation to virtio-blk dataplane, which push the data
>> >> handling out of biglock. And it is a try to implement this process
>> >> in userspace, while vhost-net in kernel.
>> >
>> > What's the motivation for doing this?
>> 
>> I haven't looked carefully at the patches, but I don't think we should
>> "cheat" when it comes to virtio-net.  I think it's useful to have a
>> baseline proof of concept but I think we should focus on making the
>> network layer re-entrant.
>> 
>> In terms of why even bother, if we can make virtio-net a viable
>> alternative to vhost-net, it's a huge win.  Being in userspace is a huge
>> advantage.
>> 
>> I understand the challenges with zero-copy but we really need a proper
>> userspace implementation to determine how much it really matters.
>> Zero-copy tap is also not entirely outside the realm of possibility.
>
> This is exactly what we have, vhost-net is the interface we use
> for asynchronous communication. If you want to add yet another
> asynchronous interface in kernel, it's certainly doable but then what's
> the point?
>
>> >> The iperf's result show it improving the perfermance of base line,
>> >> but still slower than vhost-net (maybe the rx path need optimized).
>> >> 
>> >> Todo:
>> >> implement fine lock for net-subsystem
>> >
>> > vhost-net is currently the only way to get zero copy transmit
>> > on linux (and zero copy receive in the future)
>> > so at least in theory it'll always be faster.
>> 
>> It might always be faster.  But that doesn't mean we should limit the
>> performance of virtio-net in userspace.  Some people may be willing to
>> take a small performance hit in order to obtain the security offered by
>> being in userspace.
>> 
>> Regards,
>> 
>> Anthony Liguori
>
> By 'being in userspace' I presume you mean the ring processing code.

I mean that:

1) the same code path is used regardless of network backend

2) QEMU has the ability to manipulate the traffic if it needs/wants to

3) the state of the device model is userspace at all times

4) QEMU is always the front-line interface communicating with the guest

Yes, this could be a model where -netdev vhost-net was the backend used
by QEMU.

> Ring processing can be done in userspace if you are ready
> to use the synchronous read/write operations, skipping
> this bunch of code might improve security. But it requires a data
> copy almost by definition.

Not sure what you mean here.  If you are talking about read/write on a
tap file descriptor, I'm not even suggesting that.

I'm perfectly fine with vhost-net being a -netdev backend.  But that's a
very different model than how we use it today.

Regards,

Anthony Liguori

>
>
>> >
>> >> Liu Ping Fan (9):
>> >>   vring: split the modification and publish of used ring
>> >>   vring: introduce vring_restore() to restore from img
>> >>   event poll: make epoll work for normal fd
>> >>   event poll: pass event type to event callback
>> >>   event poll: enable event poll handle more than one event each time
>> >>   virtio net: introduce dataplane for virtio net
>> >>   tap: make tap peer work on dedicated data-plane thread
>> >>   virtio net: enable dataplane for virtio net
>> >>   configure: make virtio net dataplane configurable
>> >> 
>> >>  configure                  |   12 ++
>> >>  hw/dataplane/Makefile.objs |    4 +-
>> >>  hw/dataplane/event-poll.c  |   69 +++++--
>> >>  hw/dataplane/event-poll.h  |   14 ++-
>> >>  hw/dataplane/virtio-blk.c  |    8 +-
>> >>  hw/dataplane/virtio-net.c  |  444 
>> >> ++++++++++++++++++++++++++++++++++++++++++++
>> >>  hw/dataplane/virtio-net.h  |   32 ++++
>> >>  hw/dataplane/vring.c       |   37 ++++
>> >>  hw/dataplane/vring.h       |    3 +
>> >>  hw/virtio-net.c            |   94 +++++-----
>> >>  hw/virtio-net.h            |   64 +++++++
>> >>  hw/virtio-pci.c            |    2 +-
>> >>  include/net/tap.h          |    6 +
>> >>  net/tap.c                  |   92 +++++++++-
>> >>  14 files changed, 806 insertions(+), 75 deletions(-)
>> >>  create mode 100644 hw/dataplane/virtio-net.c
>> >>  create mode 100644 hw/dataplane/virtio-net.h
>> >> 
>> >> -- 
>> >> 1.7.4.4
>> >>